This invention relates to an antenna system; particularly, but not exclusively, to a multiple beamformer for a satellite mobile communication system.
Such systems are described (in general terms) in, for example, WO93/09029; WO93/09577; WO93/09578; WO93/09613; WO93/09614; WO93/09624; EP-A-0510789; EP-A-035885; EP-A-421722. The proposed Inmarsat (TM) P21 system consists of a number of earth stations, which may be mobile stations, having antennas for communication with a constellation of communications satellites. Each satellite carries an antenna system designed to transmit and receive an array of multiple beams, each directed towards a portion of the surface of the Earth, the beams together covering the entire surface of the Earth.
Each beam carries a number of frequency-multiplexed channels; for example, the bandwidth of each beam may be 5 MHz, enabling each to carry a large number of user channels (typically carrying voice telecommunications). The satellite typically also carries an antenna for connection to a fixed earth station, communicating for example with a public telecommunications network.
To synthesize a plurality of beams in the far field beam pattern, if the transmit and receive antennas are directly radiating antennas (i.e. without reflectors) consisting of a large array of radiating elements, a conventional beam forming network requires, in principle, a phase shifter for every radiating element, for each beam position, and a power divider for every beam. Thus, for one hundred elements and on hundred beams, 10,000 phase shifters and 100 power dividers are needed, and the number of components grows roughly exponentially for large numbers of beams and elements.
This represents a considerable weight of RF components, and the power losses of the feeding system is also high. Both weight and electrical power consumption are at a premium in satellites.
One alternative type of beam former for array antennas is the xe2x80x9cButler matrixxe2x80x9d, described in U.S. Pat. No. 3,255,450 (Butler), which consists of a butterfly cascade arrangement of four-port power dividers with associated phase shifters, receiving N input RF signals and feeding a linear array of N spaced elements. The dividers each receive two input analogue RF signals, one of which is phase shifted, and output two RF signals with a 90 degree phase difference. The effect of the array of dividers and phase shifters is that the RF signal supplied to any one of the inputs is fed, in progressively incrementing phase shifts, to each of the elements of the array. Thus, the array acts as a phased array, generating a beam at an angle dependent upon the phase shift increment (which depends upon the number of radiating elements) and the element spacing.
By selectively exciting each input in turn, an incrementally scanning beam can be generated which may be used in radar applications. Alternatively, the beamformer can be used to generate a grid of multiple fixed beams from a common aperture. A beam can be scanned in one of two orthogonal directions by providing several such linear matrices in aligned rows and columns, the outputs of the row matrices feeding the inputs of the column matrices, and the outputs of the column matrices feeding a two dimensional array of radiators.
Butler matrices are virtually lossless, and this tends to be the reason for their use.
JP-A-59-44105 discloses a two-dimensional beam-forming network comprising two orthogonal stacks of Butler matrices, for forming beams lying aligned along angles on a rectangular array. EP-A-0056205 discloses a large Butler matrix formed from two orthogonal stacks of Butler matrices.
WO88/04837 discloses a steerable beam reflector antenna used on a communications satellite in which a Butler matrix is used for beam steering.
EP-A-0468662 discloses an antenna (which may be a directly radiating antenna) in which a Butler matrix is used as a power splitter to distribute power between antenna array elements to form a single unidirectional composite beam, the progressive phase shifts provided by the matrix being cancelled by phase shifter elements.
A feature of linear Butler matrix array antennas is that the crossover point between adjacent beams is over 3dB down, so that the power between the beams drops off to half the maximum beam level. For a square array, the power minima between 4 adjacent beams are 8dB down, which would, of itself, render a conventional square Butler matrix unsuitable for forming multiple satellite communication beams, since it is desirable to provide uniform coverage of the Earth surface.
According to the present invention, there is provided an antenna system using a passive power splitter matrix (e.g. a Butler matrix) as a beam former for a hexagonal array antenna to create a hexagonal array of beam directions.
The use of a hexagonal array gives a better coverage of the Earth surface than would an equivalent square array, since the power between adjacent beams does not drop off so deeply.
Preferably, the aperture around the array is smoothed, which, in the far field pattern, reduces the drop off in power between adjacent beams.
Preferably, the matrix is made redundant, and only some output ports are connected to radiating elements; the other output ports are terminated.
This aspect of the invention causes the Butler matrix to no longer achieve its usual advantage of being lossless. However, we have found that the loss is tolerable, for an improvement in power drop off at the crossover.
In a preferred embodiment, the amplification or loss in the path to the radiating elements differs across the aperture of the array, so as to provide a gentle taper in the power fed to the edges of the array. This aspect of the invention raises the cross-over level between beams and reduces the side-lobe level of the far-field radiation pattern.
Preferably, the matrix comprises two orthogonally connected stacks of power splitter matrices.
In another aspect, the invention provides a beam forming network for an array antenna system which comprises first and second orthogonally connected stacks of power splitter matrices, there being fewer matrices in at least one stack than the order of the matrices in that stack. Thus, xe2x80x9coversizedxe2x80x9d matrices may be employed to form non-rectangular antenna arrays, but without requiring matrices to the number of twice the order of each matrix.
In another aspect, the invention provides an antenna system in which several different power splitter matrices are provided, and corresponding output ports of each matrix are connected jointly to elements of an array antenna, so that a single array antenna can generate multiple grids of beams. By phasing the outputs of the matrices differently, the different grids can be steered to offset positions, so that one grid can be interpolated at minima between beams of another.
In another aspect, the invention provides a communication transceiver station (for example, a satellite) having a digital processor for performing channelization (i.e. multiplexing and demultiplexing) connected via an analogue beam former comprising a passive power splitting network (e.g. a Butler matrix). This enables the load on the processing device to be greatly reduced, without the substitution of a highly complex beam forming structure, and thus reduces the mass, power consumption and volume of the signal processing system, and hence makes it more suitable for use in a satellite.
In another aspect, the invention provides an antenna system in which several different Butler matrix devices are connected in parallel to the same array antenna, each device being arranged to generate an array of beam directions, the arrays being mutually offset so as to produce a combined array of beam directions having a smaller angular spacing.
Thus, a single antenna can be used to generate a large number of beams, with improved beam coverage and reduced dropoff between beams as compared to a beam array producible from a single Butler matrix device.
Other aspects and embodiments of the invention are as described in the following description and claims.